1. Field of the Invention
This invention relates to a digital-to-analog converter and it relates, in particular, to such converters for producing a so-called baseband analog signal.
2. Description of the Prior Art
In the copending application of J.C. Candy Ser. No. 412,296, filed Nov. 2, 1973, now U.S. Pat. No. 3,893,102, entitled "Digital-to-Analog Converter Using Differently Decoded Bit Groups," and assigned to the same assignee as the present invention, a digital-to-analog converter is taught which can be called a time-controlled, amplitude-supplementing type of converter. In that converter the most significant bit group of a digital character is employed to produce a corresponding analog signal. The least significant bit group of the same character is employed to determine a time during the character interval at which to increment the most significant bit group and thereby supplement the analog output signal amplitude. It has been found, however, that the resulting analog output signal train produced by a succession of digital characters includes the amplitude modulation effects of both bit groups and a width modulation effect due to the least significant bit group. The latter modulation introduces a noise signal component within the desired baseband analog signal band and is therefore objectionable.
Pulse width modulation noise of the type just mentioned occurs at times when the baseband signal is changing and is relatively insignificant for constant signals. It has been found that the energy content of the width modulation noise increases with frequency across the baseband being used. Thus, the noise is not as troublesome in video signal systems as it is in audio signal systems because the integrating effect of the human eye, which effect has no counterpart in the ear, suppresses the high frequency noise. However, both high and low frequency noise in the audio band are substantially equally audible.